Parallel flow evaporator with shaped manifolds

ABSTRACT

A method is provided for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator. An inlet manifold of the evaporator includes a first stage, at least one intermediate stage, and a final stage. Each stage includes an expansion chamber, a contraction chamber, and at least one heat exchange channel interconnecting the stage to an outlet manifold of the evaporator. The method includes throttling the two-phase refrigerant in the expansion chamber of each stage, flowing a portion of the throttled refrigerant through the at least one heat exchange channel to the outlet manifold, mixing and jetting the two-phase refrigerant in each contraction chamber to increase the velocity of the refrigerant, and passing the refrigerant out an exit of the outlet manifold.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuing application of U.S. patent application Ser. No. 10/987,961, filed Nov. 12, 2004, entitled “Parallel Flow Evaporator with Shaped Manifolds,” which application is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

This invention generally relates to air conditioning and refrigeration systems and, more particularly, to parallel flow evaporators thereof.

A definition of a so-called parallel flow heat exchanger is widely used in the air conditioning and refrigeration industry now and designates a heat exchanger with a plurality of parallel passages, among which refrigerant is distributed and flown in the orientation generally substantially perpendicular to the refrigerant flow direction in the inlet and outlet manifolds. This definition is well adapted within the technical community and will be used throughout the text.

Refrigerant maldistribution in refrigerant system evaporators is a well-known phenomenon. It causes significant evaporator and overall system performance degradation over a wide range of operating conditions. Maldistribution of refrigerant may occur due to differences in flow impedances within evaporator channels, non-uniform airflow distribution over external heat transfer surfaces, improper heat exchanger orientation or poor manifold and distribution system design. Maldistribution is particularly pronounced in parallel flow evaporators due to their specific design with respect to refrigerant routing to each evaporator circuit. Attempts to eliminate or reduce the effects of this phenomenon on the performance of parallel flow evaporators have been made with little or no success. The primary reasons for such failures have generally been related to complexity and inefficiency of the proposed technique or prohibitively high cost of the solution.

In recent years, parallel flow heat exchangers, and brazed aluminum heat exchangers in particular, have received much attention and interest, not just in the automotive field but also in the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry. The primary reasons for the employment of the parallel flow technology are related to its superior performance, high degree of compactness, structural rigidity and enhanced resistance to corrosion. Parallel flow heat exchangers are now utilized in both condenser and evaporator applications for multiple products and system designs and configurations. The evaporator applications, although promising greater benefits and rewards, are more challenging and problematic. Refrigerant maldistribution is one of the primary concerns and obstacles for the implementation of this technology in the evaporator applications.

As known, refrigerant maldistribution in parallel flow heat exchangers occurs because of unequal pressure drop inside the channels and in the inlet and outlet manifolds, as well as poor manifold and distribution system design. In the manifolds, the difference in length of refrigerant paths, phase separation, gravity and turbulence are the primary factors responsible for maldistribution. Inside the heat exchanger channels, variations in the heat transfer rate, airflow distribution, manufacturing tolerances, and gravity are the dominant factors. Furthermore, the recent trend of the heat exchanger performance enhancement promoted miniaturization of its channels (so-called minichannels and microchannels), which in turn negatively impacted refrigerant distribution. Since it is extremely difficult to control all these factors, many of the previous attempts to manage refrigerant distribution, especially in the parallel flow evaporators, have failed.

In the refrigerant systems utilizing parallel flow heat exchangers, the inlet and outlet manifolds or headers (these terms will be used interchangeably throughout the text) usually have a conventional cylindrical shape. When the two-phase flow enters the header, the vapor phase is usually separated from the liquid phase. Since both phases flow independently, refrigerant maldistribution tends to occur.

If the two-phase flow enters the inlet manifold at a relatively high velocity, the liquid phase (i.e. droplets of liquid) is carried by the momentum of the flow further away from the manifold entrance to the remote portion of the header. Hence, the channels closest to the manifold entrance receive predominantly the vapor phase and the channels remote from the manifold entrance receive mostly the liquid phase. If, on the other hand, the velocity of the two-phase flow entering the manifold is low, there is not enough momentum to carry the liquid phase along the header. As a result, the liquid phase enters the channels closest to the inlet and the vapor phase proceeds to the most remote ones. Also, the liquid and vapor phases in the inlet manifold can be separated by the gravity forces, causing similar maldistribution consequences. In either case, maldistribution phenomenon quickly surfaces and manifests itself in evaporator and overall system performance degradation.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, a method is provided for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator. An inlet manifold of the evaporator includes a first stage, at least one intermediate stage, and a final stage. Each stage includes an expansion chamber, a contraction chamber, and at least one heat exchange channel interconnecting the stage to an outlet manifold of the evaporator. The method includes throttling the two-phase refrigerant in the expansion chamber of each stage, flowing a portion of the throttled refrigerant through the at least one heat exchange channel to the outlet manifold, mixing and jetting the two-phase refrigerant in each contraction chamber to increase the velocity of the refrigerant, and passing the refrigerant out an exit of the outlet manifold.

By another aspect of the invention, the outlet manifold of the evaporator includes a first stage, at least one intermediate stage, and a final stage. Each stage includes an expansion chamber and a contraction chamber, and the heat exchange channels interconnect the stages in the inlet manifold to the stages in the outlet manifold.

By yet another aspect of the invention, in a parallel flow evaporator comprising an inlet manifold, an outlet manifold, and a plurality of channels fluidly connecting the inlet manifold to the outlet manifold, a method is provided for mitigating two-phase refrigerant maldistribution in the channels. The method includes the steps of partially evaporating the two-phase refrigerant through a repetitive series of stages in the inlet manifold. Each stage comprises an expansion chamber, a contraction chamber, and at least one of the channels. The method further includes the step of balancing hydraulic resistances between each stage and the associated channel.

By still another aspect of the invention, balancing resistances includes configuring the hydraulic resistance of the contraction chamber to be at least one and a half times lower than the hydraulic resistance of the associated channel.

By still another aspect of the invention, the expansion chambers of the repetitive series of stages in the inlet manifold include progressively smaller cross-sectional areas defining a cross-sectional area reduction ratio.

By still another aspect of the invention, the method for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator further includes the step of partially evaporating the refrigerant through a repetitive series of stages in the outlet manifold. The stages comprise an expansion chamber, a contraction chamber, and at least one of the channels.

By still another aspect of the invention, the expansion chambers of the repetitive series of stages in the outlet manifold comprise progressively larger cross-sectional areas.

By still another aspect of the invention, the progressively larger cross-sectional areas of the expansion chambers in the outlet manifold are proportional to the progressively smaller cross-sectional areas of the expansion chambers in the inlet manifold.

In the drawings as hereinafter described, preferred and alternate embodiments are depicted; however, various other modifications and alternate designs and constructions can be made thereto without departing from the true spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a parallel flow heat exchanger in accordance with the prior art.

FIG. 2 is a schematic illustration of one embodiment of the present invention.

FIG. 3 is a schematic illustration of an alternative embodiment of the present invention.

FIG. 4 is a schematic illustration of yet another embodiment of the present invention.

FIG. 5 is a schematic illustration of still another embodiment of the present invention.

FIG. 6 is a schematic illustration of still another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a parallel flow heat exchanger is shown to include an inlet header or manifold 11, an outlet header or manifold 12 and a plurality of parallel channels 13 fluidly connecting the inlet manifold 11 to the outlet manifold 12. Generally, the inlet and outlet manifolds 11 and 12 are cylindrical in shape, and the channels 13 are usually tubes (or extrusions) of flattened or round shape. Channels 13 normally have a plurality of internal and external heat transfer enhancement elements, such as fins. For instance, external fins, disposed therebetween for the enhancement of the heat exchange process and structural rigidity are typically furnace-brazed. Channels 13 may have internal heat transfer enhancements and structural elements as well.

In operation, two-phase refrigerant flows into the inlet opening 14 and into the internal cavity 16 of the inlet header 11. From the internal cavity 16, the refrigerant, in the form of a liquid, a vapor or a mixture of liquid and vapor (the latter is a typical scenario) enters the tube openings 17 to pass through the channels 13 to the internal cavity 18 of the outlet header 12. From there, the refrigerant, which is now usually in the form of a vapor, passes out the outlet opening 19 and then to the compressor (not shown).

As discussed hereinabove, it is desirable that the two-phase refrigerant passing from the inlet header 11 to the individual channels 13 do so in a uniform manner (or in other words, with equal vapor quality) such that the full heat exchange benefit of the individual channels can be obtained and flooding conditions are not created and observed at the compressor suction (this may damage the compressor). However, because of various phenomena as discussed hereinabove, a non-uniform flow of refrigerant to the individual channels 13 (so-called maldistribution) occurs. In order to address this problem, the applicants have introduced design features that will create a mixing and jetting effects in the two-phase refrigerant flow in the inlet manifold 11 to thereby bring about a more uniform homogeneous flow into to the channels 13.

Referring to FIG. 2, the heat exchanger is formed with a conventional outlet manifold 12 and channels 13, but with a differently shaped inlet manifold 21, as shown. Rather than being cylindrical in the usual manner, the inlet manifold 21 is hour-glass shaped (i.e. with a plurality of alternating expansion and contraction chambers). For simplicity, the inlet manifold 21 is shown to include three expansion chambers 22, 23 and 24 with interconnecting contraction chambers 26 and 27. Each of the expansion chambers 22, 23 and 24 is preferably interconnected to an associated channel 13, as shown. In actual practice, a larger number of expansion chambers and associated channels 13 would be provided. Further, each of the expansion chambers may be connected to more than one channel 13. It is preferred, however, that none of the channels 13 would be connected directly to a contraction chamber.

In operation, the two-phase refrigerant enters the inlet opening 28 and enters the first expansion chamber 22 where it is partially expanded with a portion thereof entering the associated channel(s). The remaining two-phase refrigerant is then forced through the contraction chamber 26 such that when it enters the expansion chamber 23 in more homogeneous manner due to increased velocity, partial evaporation (or throttling) occurs, thereby presenting a homogeneous condition for the refrigerant mixture flowing to the associated channel(s) and to the downstream channels. The remaining refrigerant then passes through the contraction chamber 27, where more mixing and jetting of the two (liquid and vapor) refrigerant phases occurs and into the expansion chamber 24, wherein, once again, a partial evaporation process is taking place, thereby presenting a homogenous mixture to the associated channel(s). In this way, the partial evaporation process is incrementally (i.e. progressively) maintained through the length of the inlet manifold 21, so as to result in a more uniform distribution of refrigerant among the channels.

Although the refrigerant flow in the inlet manifold 21 is progressively diminishing, it is essential not to introduce excessive flow impedance in the inlet manifold 21 relative to other flow resistances in the heat exchanger. Thus, the cross-section areas of the contraction chambers 26 and 27 must be properly sized for a particular application and for a particular configuration of the heat exchanger to maintain the balance between the desired partial evaporation process and undesired additional hydraulic resistance for the refrigerant flowing to the downstream channels. Generally, flow impedance of the contraction chamber should be at least one and a half times lower than the hydraulic resistance of the associated channels 13. It is also desirable to balance the impedance of the contraction chambers in the inlet manifold 21 with corresponding pressure drops in the outlet manifold as will be further described hereinafter.

An alternative embodiment of the present invention is shown in FIG. 3, wherein the inlet manifold 31 is again hour-glass shaped but with expansions chambers 32, 33 and 34 being progressively smaller in a cross section to accommodate the reduced refrigerant flow, as it moves from the inlet 38 toward the downstream end thereof. Further, the contraction chambers 36 and 37 are also preferably formed of a progressive smaller size for the same reasons. Generally, the cross-section area reduction ratio of the expansion chambers is proportional to the refrigerant flow rate ratio reduction entering and leaving the chamber. If this ratio is not uniform, the average value should be used instead for the estimates. The contraction chambers can be sized by employing an identical procedure and values.

In FIG. 4, a further embodiment is shown wherein the inlet manifold 21 is identical to that as described in respect to FIG. 2 or that shown in FIG. 3, but the outlet manifold 41 also being hour-glass shaped with expansion chambers 42, 43 and 44 and contraction chambers 46 and 47 alternately disposed as shown. These expansion and contraction chambers are not necessarily and most likely will not be of the same sizes as those of the inlet manifold 21, since the refrigerant flowing within the outlet manifold 41 in a completely different thermodynamic state. Although if the chamber of the inlet manifold 21 are progressively smaller in size toward the downstream end thereof, the chambers of the outlet manifold 41 should preferably be progressively larger toward the downstream end thereof, as shown in FIG. 5, and identical aspect ratio can be utilized in sizing the outlet manifold chambers. In this way, the impedances that are presented in the inlet manifold 21 are matched by those in the outlet manifold 41 such that, the most favorable conditions for the uniform refrigerant flow distribution among the parallel channels 13 are created throughout the heat exchanger, enhancing the system performance and improving compressor reliability, by preventing flooded conditions at the compressor suction.

An alternative embodiment of the present invention is shown in FIG. 6, wherein the inlet manifold 51 is again hour-glass shaped with the expansions chambers 52, 53 and 54 and the contraction chambers 55 and 56 disposed in between the expansion chambers. Additionally, refrigerant-mixing inserts 57 are placed within the contraction chambers 55 and 56 to promote mixing and even more homogeneous conditions at the entrance of the adjacent downstream expansion chambers. Although inserts 57 can be spiral in shape or have internal fins or indentations, any other configurations promoting mixing are also acceptable. In all other aspects this embodiment is similar to the embodiments discussed above.

In has to be understood that the expansion and contraction chambers may be of any shape, cross-section area and configuration as long as a repetitive process of partial evaporation is created and a proper balance of hydraulic resistances is maintained.

Furthermore, it should be noted that both vertical and horizontal channel orientations will benefit from the teaching of the present invention, although higher benefits will be obtained for the latter configuration. Also, although the teachings of this invention are particularly advantageous for the evaporator applications, refrigerant system condensers may benefit from them as well.

While the present invention has been particularly shown and described with reference to preferred and alternate embodiments as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the true spirit and scope of the invention as defined by the claims. 

1. A method for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator, the method comprising the steps of: providing a two-phase refrigerant to an inlet manifold of the evaporator, the inlet manifold comprising a first stage, at least one intermediate stage, and a final stage, each stage comprising an expansion chamber, a contraction chamber, and at least one heat exchange channel interconnecting the stage to an outlet manifold; throttling the two-phase refrigerant in the expansion chamber of each stage; flowing a portion of the throttled refrigerant through the at least one heat exchange channel to the outlet manifold; mixing and jetting the two-phase refrigerant in each contraction chamber to increase the velocity of the refrigerant thereby; and passing the refrigerant out an exit of the outlet manifold.
 2. The method as set forth in claim 1, wherein a plurality heat exchange channels interconnect one stage to the outlet manifold, and the step of flowing a portion of the throttled refrigerant includes flowing through the plurality of heat exchange channels.
 3. The method as set forth in claim 1, wherein the at least one heat exchange channel is fluidly connected to the expansion chamber.
 4. The method as set forth in claim 1, further including the step of balancing hydraulic resistances between the contraction stage and the associated heat exchange channel.
 5. The method as set forth in claim 4, wherein the hydraulic resistance of the contraction chamber is at least one and a half times lower than the hydraulic resistance of the associated channel.
 6. The method as set forth in claim 1, wherein the outlet manifold further comprises a first stage, at least one intermediate stage, and a final stage, each stage comprising an expansion chamber and a contraction chamber, the heat exchange channels interconnecting the stages in the inlet manifold to the stages in the outlet manifold.
 7. The method as set forth in claim 6, the heat exchange channels interconnecting the expansion chambers in the inlet manifold to the expansion chambers in the outlet manifold.
 8. The method as set forth in claim 1, wherein the mixing step comprises placing refrigerant-mixing inserts in the contraction chambers.
 9. The method as set forth in claim 1, wherein the heat exchange channels are microchannels.
 10. In a parallel flow evaporator comprising an inlet manifold, an outlet manifold, and a plurality of channels fluidly connecting the inlet manifold to the outlet manifold, a method for mitigating two-phase refrigerant maldistribution in the channels, the method comprising the steps of: partially evaporating the two-phase refrigerant through a repetitive series of stages in the inlet manifold, wherein each stage comprises an expansion chamber, a contraction chamber, and at least one of the channels; and balancing hydraulic resistances between each stage and the associated channel.
 11. The method as set forth in claim 10, wherein the balancing step comprises configuring the hydraulic resistance of the contraction chamber to be at least one and a half times lower than the hydraulic resistance of the associated channel.
 12. The method as set forth in claim 11, wherein configuring the hydraulic resistance of the contraction chamber comprises sizing a cross-sectional area of the contraction chamber.
 13. The method as set forth in claim 10, wherein a refrigerant flow rate progressively decreases through the repetitive series of stages in the inlet manifold.
 14. The method as set forth in claim 13, wherein the expansion chambers of the repetitive series of stages in the inlet manifold comprise progressively smaller cross-sectional areas defining a cross-sectional area reduction ratio, the ratio being proportional to the progressively decreasing refrigerant flow rate.
 15. The method as set forth in claim 13, wherein the contraction chambers of the repetitive series of stages in the inlet manifold comprise progressively smaller cross-sectional areas defining a cross-sectional area reduction ratio, the ratio being proportional to the progressively decreasing refrigerant flow rate.
 16. The method as set forth in claim 10, further including the step of partially evaporating the refrigerant through a repetitive series of stages in the outlet manifold, wherein the stages comprise an expansion chamber, a contraction chamber, and at least one of the channels.
 17. The method as set forth in claim 16, wherein a refrigerant flow rate progressively increases through the repetitive series of stages in the outlet manifold.
 18. The method as set forth in claim 16, wherein the expansion chambers of the repetitive series of stages in the inlet manifold comprise progressively smaller cross-sectional areas, and the expansion chambers of the repetitive series of stages in the outlet manifold comprise progressively larger cross-sectional areas.
 19. The method as set forth in claim 18, wherein the progressively larger cross-sectional areas of the expansion chambers in the outlet manifold are proportional to the progressively smaller cross-sectional areas of the expansion chambers in the inlet manifold.
 20. The method as set forth in claim 10, wherein the heat exchange channels are microchannels. 